In this app note we will show you how to equalize all channels of your home theater system using the miniDSP nanoAVR. Before starting on this app note, you will need to connect the nanoAVR into your system and ensure that it is working correctly. To do so, please read through the User Manual.
- Why equalize?
- What you will need
- 1. Getting set up for measurement
- 2. Set your routing and downmix
- 3. Establish your baseline
- 4. Equalize the front left and right speakers
- 5. Equalize the center and surround speakers
- 6. Equalize the LFE/subwoofer channel
- What's next?
Why equalize? [Top]
For the purposes of home theater, equalization can be divided into two areas. Subwoofer equalization is used to smooth the low-frequency response of the system, and covers the frequency spectrum from 20 Hz (or lower) up to about 120 Hz, depending on the specifics of the system. The goal is greater accuracy: what you hear at the listening position is closer to what the recording engineer intended.
Full-range equalization covers the frequency range above that handled by the subwoofer. It compensates for response variations caused by acoustic reflections and by compromised speaker placement, such as against a wall or into a cabinet. Center and surround speakers in particular are prone to response errors caused by design and placement limitations. Equalizing all speakers in a home theater system to have the same in-room frequency response will ensure optimum creation of soundstage and envelopment.
What you will need [Top]
Room EQ Wizard (REW). Be sure to download the latest version from the Downloads Area for UMIK-1 support.
A microphone stand with boom arm, available in music supply stores. (While the UMIK-1 is supplied with a small "table top" stand, best results will be obtained if the microphone is placed in free space, away from sofas or tables.)
1. Getting set up for measurement [Top]
The diagram below illustrates a typical setup used for acoustic measurement. The computer can be connected to HDMI Input #2, so there is no need to disconnect the player connected to HDMI Input #1. The computer will need an HDMI output port, or if you have a Mac with a Thunderbolt port, then a Thunderbolt to HDMI adapter cable can be used.
Setting up the computer to use REW with HDMI output and UMIK-1 input is covered in these app notes:
2. Set your routing and downmix [Top]
To start with, go to the Routing screen and ensure that you have a straight-through routing with no bass management. Here is how it looks:
You want the AVR to pass the signal through unaltered, so:
- Turn off all bass management and down-mixing. Typically this is done by setting all speakers to "large" (even the ones that don't exist).
- Set all channels to the same level (typically, 0 dB).
- Turn off all forms of matrix decoding (e.g. Dolby Pro Logic, DTS Neo:6) and any surround simulation or enhancement modes.
Now confirm that you are able to play a test signal through each HDMI channel in turn. A useful tool here is the REW signal generator set to play pink noise (pink noise is easier to localize than a pure sine wave):
Set the volume control on your AVR fairly low to start with, and use the HDMI output channel selector (REW Preferences on Windows, SoundFlowerBed on Mac) to send audio to channel 1. You should hear a hissing sound from the front left speaker. Then use the HDMI channel selector to proceed through each channel in turn.
If you have less than a full suite of 7.1 speakers, you will need to "downmix" some channels. (While your AVR can do this, equalization is more straightforward if the nanoAVR is set to do it.) For example, if you have only two surround speakers, mix the rear left input to the left surround output and the rear right input to the right surround output, like this:
If you don't have a center channel speaker — that is, you are running a "phantom center" — mix the center channel input to the front left and front right outputs at a level of -3 dB, like this:
3. Establish your baseline [Top]
Use REW to measure the in-room frequency response of each of your speakers and subwoofer, one at a time. (You don't need to run a measurement on channels that are down-mixed to other channels.) For best results, point the microphone towards the speaker being measured.
You will end up with a set of up to eight frequency response measurements. Use the REW Overlay window and 1/3rd, 1/6th, or 1/12th octave smoothing to view them. Here is an example showing the left channel in blue, the center channel in green, and the subwoofer in red. (The other speakers are omitted from the example to avoid cluttering the graph.)
Performing this baseline measurement ensures that your setup is ready to proceed with equalization. In some cases, the baseline measurements may reveal issues that will need to be corrected before proceeding. For example, if the subwoofer response has a deep notch, no amount of equalization will be able to correct it and you should try some other positions of the subwoofer in the room.
4. Equalize the front left and right speakers[Top]
Start by equalizing the front left and right speakers. These are usually the highest quality speakers in the system and have fewest placement problems. The goal when equalizing these speakers with an in-room measurement is to obtain a fairly even response that gently slopes down from low frequencies to high frequencies.
To perform an automatic equalization with REW, follow the steps detailed in the app note Auto-EQ tuning with REW. You will need to alter the settings slightly, as follows:
Important items to set are the MiniDSP-96k button (the nanoAVR operates internally at 96 kHz), the settings related to the HF Fall, and the Match Range frequencies.
You may also be interested in trying "manual" equalization. This is where you set the EQ settings directly in the nanoAVR user interface. It is more time-consuming but gives you more control over the final result. You can do this in a different configuration if you like, so you can easily switch between them to compare. Link the PEQ filters for the front left and right channels (generally, it's best to avoid using different EQ settings for the left and right speakers):
The sequence when doing manual equalization is 1. measure, 2. perform an EQ adjustment, 3. measure again. Of the various types of EQ available in the nanoAVR interface, there are two that we suggest as the ones to start with. The first is the PEAK type filter with a negative gain. Use this to "flatten" peaks in the measured response. Here is an example setting:
The second is a high-shelf filter. One use of this type of filter is to correct for the response of the speaker in the mid-range, where placement may alter the designed response. The center frequency is typically a few hundred Hz and the gain may be positive or negative, depending on your particular acoustic situation:
The second use is a gentle high-frequency slope to adjust the "tilt" of the in-room frequency response curve at high frequencies:
You may find that the left and right speakers measure a bit differently. In that case, you will need to pick a compromise. Here are some additional guidelines:
Use boost filters sparingly. If used, they should have a fairly low Q (1 or less) and a moderate amount of gain (4-5 dB max). Don't try and use a narrow boost to fill in a deep notch in the response.
View measurements at 1/3rd octave smoothing as a general rule. Switch to 1/12th octave smoothing to check that you are not attempting to correct a narrow dip or peak with a broad filter.
Even with negative gain, use high-Q filters carefully. If there is a persistent resonance that you can measure in different locations in the listening room, it may be an architectural issue (such as a corridor or stairwell), in which case a narrow filter can help.
Avoid using EQ to increase the amount of bass from your speakers — leave that to the subwoofer. Your speakers have a natural low-frequency rolloff and trying to change this by too much is a recipe for overdriving amps and woofers.
Make some additional measurements with the microphone in different locations around your seating area. This can be used as a "sanity check" to ensure that you are not equalizing for issues that vary a lot over the seating area and that you are not "over-equalizing."
This plot shows the front left channel of our example system before any EQ in light blue and afterwards in dark blue:
5. Equalize the center and surround speakers[Top]
Now move on to the center and surround speakers. The goal with these is not only to even out the frequency response, but to also get approximatelythe same in-room frequency curve as the front left and right speakers. You may find that high-shelf and low-shelf filters are particularly useful for this purpose. Also, use the gain adjustment on these channels so that the levels match the front speakers.
This plot shows the result of equalizing the center channel speaker in our example system compared to the front left speaker (the center is in green, the front left in blue):
Due to placement issues, the frequency response measurements of the center and surround speakers, even after equalization, will likely not be as smooth as the front left and right speakers. This is OK — you don't need to get the curves perfectly smooth, just get to a reasonable compromise.
6. Equalize the LFE/subwoofer channel[Top]
To equalize the LFE/subwoofer channel, use the auto-EQ function of REW. This is described in detail in the app note Auto-EQ tuning with REW, so that procedure should be followed carefully. The key difference is that the Equalizer setting must be set to "miniDSP-96k", since the nanoAVR operates internally at 96 kHz.
As the final step, adjust the level of the subwoofer so that the LFE channel measures 10 dB higher than the speaker channels. This plot shows the LFE/subwoofer channel response in our example system before equalization in orange and afterwards in red, with the front left speaker in blue used as the reference for setting the subwoofer level:
You may also want to make use of the target curve feature of REW to create a response that is more emphasized at lower frequencies:
Remember also that the better the response of your subwoofer before equalization, the better the result will be afterwards. Subwoofer response is very sensitive to placement within the room, so experiment with different subwoofer locations in order to achieve the best final result.
What's next? [Top]
If you have full-range (20 Hz to 20 kHz) speakers all around (front left and right, center, and all surrounds), then your job is done! You can sit back and listen. In practice, very few home theaters have a full suite of full-range speakers, and so bass management is required to send low frequency content to the subwoofer. We will cover this in the app note Bass Management with the nanoAVR.
You can continue to fine-tune your system while listening. For example, set up some high-frequency shelving filters to get the overall tonal balance that you prefer. Adjust the level of the subwoofer channel to get the best balance of bass. And so on. In the end, it's really all about how the system sounds to you. Have fun!